Methods for manufacturing optical fiber preforms are known.
For example, U.S. Pat. No. 4,741,747, which is hereby incorporated by reference in its entirety, discloses a method of fabricating optical fibers, wherein an end of a preform (i.e., end taper) is reduced by moving a plasma in the area of at least one reversal point nonlinearly with time and/or by changing the longitudinal extent of the plasma as a function of time. The term “end taper” means the deposition regions at the ends of the substrate tube, where the optical and geometric properties of the deposited layers are insufficiently constant.
U.S. Pat. No. 5,188,648, which is hereby incorporated by reference in its entirety, discloses a method for reducing the geometric end taper by interrupting the reciprocating movement of the plasma at the reversal point on the supply side of the glass-forming gases during glass deposition.
U.S. Pat. No. 5,145,509, which is hereby incorporated by reference in its entirety, discloses a method for reducing the geometric taper, by placing a glass rod inside the substrate tube, such that the longitudinal axis of the glass rod coincides with that of the glass tube, and the radius of the glass rod is selected to be a maximum of 0.67 times and a minimum of 0.2 times the inner radius of the glass substrate tube. The glass rod is removed from the interior of the substrate tube upon completion of the deposition process, followed by the collapsing of the hollow substrate tube into a solid preform at an elevated temperature.
International Patent Application No. WO 2004/101458 and its counterpart U.S. Pat. No. 7,068,899, which is hereby incorporated by reference in its entirety, discloses a method for reducing the taper in a preform, by varying the velocity of the plasma in a first “end region” near a reversal point, both as a function of time during the deposition process, and as a function of the position in the first end region. The term “end region” is defined as the region in which the velocity of the plasma is varied as a function of the position.
One of the problems of the methods according to the above-cited documents is the fact that an optimization of the geometric taper will lead to optical taper and vice versa. A Plasma Chemical Vapor Deposition (PCVD) process, for example, carries an inherent tradeoff between geometric taper and optical taper.
Accordingly, there is a need for an improved method for making optical preforms by way of a vapor deposition process.